The present invention relates to improved lead frame, and more particularly relates to lead frame having an improved composition and well suited for use for IC elements and transistors.
Conventionally, Fe-Ni alloys or Fe-Ni-Co alloys are in general known as the material for such lead frames. As one of the Fe-Ni-Co alloys used for this purpose, the alloy "F-15" of ASTM is known which roughly contains 54% by weight of Fe, 29% by weight of Ni and 17% by weight of Co. As another of the Fe-Ni alloys used for this purpose, the alloy "F-30" of ASTM is known which contains about 42% by weight of Ni.
However, the relatively high content of costly Ni and Co in these alloys has the disadvantage of high material cost. In addition, these alloys are unsuited for the silver plating which is to be carried out in manufacturing of lead frames. In order to eliminate this difficulty in silver plating, Ni plating has to be applied to the object as a pre-treatment for the silver plating by the so-called strike plating methods, i.e. plating with high electric current density for a short period. Such a double staged plating process naturally causes a rise in manufacturing cost of lead frames.
Copper or copper alloys are also used for manufacturing of lead frames. These copper materials, however, are in general inferior to the above-described Fe-Ni alloys and Fe-Ni-Co alloys in mechanical strength. Such lower mechanical strength of the alloys tends to form a bar to smooth and trouble-free mounting of complete IC elements to printed circuit base boards. In such mounting, a lead or leads of each IC element are forcedly inserted into associated apertures in a printed circuit base board by means of an automatic machine. Consequently, low mechanical strength of the material alloy tends to cause undesirable buckling of the leads and/or unsuccessful insertion of the leads.
Low-carbon steel is known as a material which is cheaper than Fe-Ni alloys and Fe-Ni-Co alloys but provided with appreciably high mechanical strength. Therefore, it is within consideration to use low-carbon steel for manufacturing of lead frames. However, the conventional low-carbon steels are in general very low in anticorrosion property and, consequently, quite unsuited for use in manufacturing of lead frames. In manufacturing of lead frames, an elongated material plate is shaped into a corrugated form by a press and silver plating is applied to the surface of the corrugated form. After connection to a chip of each IC element, packaging is carried out to form respective IC elements. Finally, cutting is effected on the lead frames. Apparently, the cut ends of the lead frames are not plated with silver. Thus, when a low anti-corrosion material is used for the material plate, stain is liable to start at the cut ends of the lead frames. Further, when the corrugated material plate is left untreated before the silver plating, development of stain on the surface of the plate tends to hinder ideal plating of the surface. For these reasons, it is quite unfeasible to use low-carbon steel for manufacturing of lead frames.